In recent years, UWB (Ultra Wideband) communication technology has attracted attention as next-generation wireless-communication technology. The UWB communication technology is a high-speed wideband communication technology of a spread spectrum type using a radio wave with a large fractional bandwidth. The UWB communication technology can be used for a high-speed indoor multi-points-connection radio communication method.
As a method of generating a signal used for the UWB communication, there is a method of transmitting a continuous chain of impulses with a short duration, directly from an antenna. The UWB communication method using such a continuous chain of impulses is called a UWB-IR (Ultra Wideband-Impulse Radio) method. The continuous chain of impulses is hereinafter called a repetitive pulse train.
Document 1 (Japanese translation of PCT international application H10-508725) discloses, as an example of the UWB-IR method, an art which transmits data by transmitting a series of pulses with a duration in nanoseconds without using a carrier wave. The art has a feature of transmitting a signal with a transmitting level lower than an environmental noise level over an extremely wide frequency band; thereby the art can reduce electric power consumption as compared to the conventional radio communication with a carrier wave. Since the ultra short pulse is used, the art possesses such advantages that the art enables high-speed communications and is strong against multi-pass interference.
In the UWB-IR method, information is put on a repetitive pulse train to be sent. It is considered that the UWB-IR method uses a repetitive code because the UWB-IR transmits repetitively a plurality of pulses for one bit of information bit train. Document 2 (Naotake Yamamoto and Tomoaki Otsuki; “Evaluation of Characteristics of Internally Turbo-Coded Ultra Wideband-Impulse Radio (ITC-UWB-IR) Method”, Institute of Electronics, Information and Communication Engineers, technical report RCS2002-55, pp. 25-30, May 2002.) has proposed “an internal turbo code UWB-IR method” as a method incorporating an error correcting code instead of the repetitive code. The error correcting code incorporated is considered more powerful than the repetitive code.
FIG. 16 is a block diagram of the conventional UWB transmitting device, and shows in detail a transmitter part of “the internal turbo code UWB-IR method” which is disclosed in Document 2.
As shown in FIG. 16, the conventional UWB transmitting device comprises an encoder 1, a serial-to-parallel converter 2, a pulse generator 3, a parallel-to-serial converter 4 and an antenna 5. An information bit train from an information signal source S is encoded to an n-bit serial turbo encoded bit train by the encoder 1, and is converted into an n-bit parallel encoded bit train by the serial-to-parallel converter 2. The pulse generator 3 has n-piece repetitive pulse generators 3_1-3_n, inputs n-bit parallel coded bits, and outputs n-piece pulse trains in parallel. Each of the n-piece pulse trains comprises tens of to hundreds of repetitive pulses which have been generated corresponding to each coded bit. The n-piece pulse trains are parallel-to-serial converted by the parallel-to-serial converter 4, and are directly transmitted from the antenna 5.
In the conventional UWB transmitting device shown in FIG. 16, when Ns-piece pulses in total are transmitted repetitively per one bit of the information bit train, each of the n sets of repetitive pulse generators 3_1 to 3_n generates (Ns/n)-piece repetitive pulses, respectively.
FIG. 17 is a block diagram of the conventional UWB receiving device, and shows in detail a receiver part of “the internal turbo code UWB-IR method” which is disclosed in Document 2.
As shown in FIG. 17, the conventional UWB receiving device comprises the antenna 5, a pulse wave-shape correlator 6, a pulse train integrator 7, a decoder 8 and a decision circuit 9. As for received pulses received by the antenna 5, correlation with a template wave shape is taken in the pulse wave-shape correlator 6. In the pulse train integrator 7, the correlation values are integrated as many as the number of the repetitive pulses. After a soft decision of a code is made in the decoder 8 which decodes a turbo code using the integrated correlation value, a hard decision is made, and an information bit train is restored and outputted as a decoded information signal in the decision circuit 9.
According to the conventional technology with the internal turbo code disclosed in Document 2, error rate characteristics can be improved without reducing transmission speed as compared with the UWB-IR method, by controlling the coded rates (1/n) in the encoder and the number of repetitive pulses (Ns) of the UWB-IR method, depending on a state of communication path or required quality.
In the above-mentioned conventional technology, the equal number of pulses as the (Ns/n)-piece of repetitive pulses are generated repetitively to each bit of the n-bit encoded bit train; therefore significance of every encoded bit is equal. That is, the conventional technology mentioned above does not take into consideration changing in the significance of the encoded bits adaptively in consideration of the state of the communication path; hence, measures against changes of the state of the communication path are insufficient. Accordingly, adaptive measures are difficult to be performed, by allotting many repetitive pulses to an encoded bit which is susceptible to adverse effect by noises and interference from other users, or by allotting, on the other hand, less repetitive pulses to an encoded bit which is hard to be influenced by the adverse effect. Furthermore, there is restriction that the number of repetitive pulses which the pulse generator 3 generates must always be a multiple of “n” when the coded rate of the encoder 1 is (k/n).